*** Project Update: The project failed.... As stated before, I am not an electrical engineer. The amperage requirements were just too much for this setup. Oh well, better luck next time.

Traxxas EZStart System

Many in the hobby find the EZStart system to be less than desirable. I have only been in the sport for a short time so my opinion does not carry much weight, but I find it to be convenient. I have only had one issue with the EZStart system and that is when some dirt got into the electric motor and prevented it from spinning. I tried compressed air but the only way to get it out was to disassemble the motor (Part 5279); which it is not permitted by it's design. I was able to successfully pull the motor apart and reassemble it, but it is above the skill level of a novice.

Last summer I installed a 12v outlet in the back of my Nissan Frontier to use while camping. So this project is to make use of that outlet. When I go bashing it is usually from the back of my truck. I purchased a second EZStart wand on eBay for $6 that I will be adding a 12v cigarette lighter plug. So now I will have one wand that I can use with a battery pack and the other will be at the truck. The truck outlet is obviously 12 volts and will need to be regulated to allow only 7.2 volts. I will be doing this by using an adjustable voltage regulator (Radio Shack Part# 276-1778)

I am not an electrical engineer but I know I can figure this out. The battery pack I am currently using is 7.2v 1800mAh (mAh means Ampere Hour). Using the adjustable voltage regulator (LM317T) I will be able to safely lower the voltage for the EZStart wand to 7.2v.

Definitions

Ampere, or Amp The unit of measurement of current flow. One volt placed across a one ohm resistance will cause a current of one Amp to flow.

Ampere-hour or AH The unit of electrical capacity - this tells you how much power the battery will store. Current multiplied by time in hours equals ampere-hours. A current of one amp for one hour would be one amp-hour; a current of 3 amps for 5 hours would be 15 AH. Similar to the "gallons per day" measure of water. Amp-hour ratings will vary with temperature, and with the rate of discharge. For example, a battery rated at 100 AH at the 6-hour rate would be rated at about 135 AH at the 48-hour rate. Ampere-hours (AH) designates the storage capacity of the battery. SLI batteries are not rated in AH, but in "CCA", or cold-cranking amps (marine batteries are often rated in "marine cranking amps").. Terms such as "6 hour rate" or "20 hour rate" indicate that the battery is discharged steadily over 6 or 20 hours, and the Amp-hour capacity is measured by how much it puts out before reaching 80% DOD.

LM317T Variable Voltage Regulator

The LM317T is a adjustable 3 terminal positive voltage regulator capable of supplying in excess of 1.5 amps over an output range of 1.25 to 37 volts. The device also has built in current limiting and thermal shutdown which makes it essentially blow-out proof. Output voltage is set by two resistors R1 and R2 connected as shown below. The voltage across R1 is a constant 1.25 volts and the adjustment terminal current is less than 100uA. The output voltage can be closely approximated from Vout=1.25 * (1+(R2/R1)) which ignores the adjustment terminal current but will be close if the current through R1 and R2 is many times greater. A minimum load of about 10mA is required, so the value for R1 can be selected to drop 1.25 volts at 10mA or 120 ohms. Something less than 120 ohms can be used to insure the minimum current is greater than 10mA. The example below shows a LM317 used as 13.6 volt regulator. The 988 ohm resistor for R2 can be obtained with a standard 910 and 75 ohm in series. When power is shut off to the regulator the output voltage should fall faster than the input. In case it doesn't, a diode can be connected across the input/output terminals to protect the regulator from possible reverse voltages. A 1uF tantalum or 25uF electrolytic capacitor across the output improves transient response and a small 0.1uF tantalum capacitor is recommended across the input if the regulator is located an appreciable distance from the power supply filter. The power transformer should be large enough so that the regulator input voltage remains 3 volts above the output at full load, or 16.6 volts for a 13.6 volt output. Source